Method and apparatus for processing coatings, radiation curable coatings on wood, wood composite and other various substrates

Abstract

A method and apparatus that combines the coating spray booth with process heating and radiation to increase the efficiency of the processing of powder coatings on wood, wood-based composite materials and plastic substrates. The temperature of the parts in process can be maintained within the booth after preheating or elevated in temperature within the booth before, during, and after the application of the coating material. The invention allows for the control of the rate of thermal expansion of heat sensitive materials, thereby reducing substrate damage from cracking. Increased efficiencies permit a significant reduction of processing energy expense. Multiple coatings can be applied and cured to parts in process within the invention while experiencing an overall reduction in the length of the processing system compared to the prior art.

Description

[0001] The present invention relates to a method and apparatus that safely combines the coating spray booth with infrared radiant process heating to increase the efficiency of processing coatings on temperature sensitive substrates, such as wood, wood-based materials, and plastics. The terms "temperature sensitive" refers to objects that possess a low rate of thermal conductivity and / or a relatively high thermal expansion factor. The application of ultraviolet radiation can also be safely combined with the spray booth equipment for efficiently processing ultraviolet curable coatings. The invention provides a method and apparatus to safely maintain, increase and control the surface temperature of objects in process while in the spray booth before, during and after the application of the coating material upon the objects. The invention also provides a method and apparatus to safely control the coefficient of thermal expansion of the objects in order to avoid physical damage to tempera...

AI Technical Summary

Benefits of technology

[0021] It should be noted that the atmosphere, including the chilled (or warm) moisture controlled fluid, does not efficiently absorb IR radiation. The cool (or warm) fluid that contains the valuable moisture for enhancing the electromagnetic attraction of the powder is not in jeopardy when applied prior to the initial IR preheat process. The IR radiation will pass freely through the cool (or warm) fluid that is lying in the grooved areas, and will be successfully absorbed by the substrate. Some heat transfer may occur from the substrate to the enhanced fluid, but primarily by thermal conduction. The heat energy transfer will be proportional to the temperatures achieved in the substrate, the temperature of the enhanced fluid, the relative humidity of the chilled (or warm) air, and the amount of time of intimate contact prior to the application of the powder coating. This invention calls for a minimum of time between radiation processing and the application of the powder, and in some cases, may be simultaneous. The IR radiation process does not endanger the strategic value of the supplemental enhanced fluid. The moisture controlled fluid will still be retained in the grooves after preheating and upon presentation of the substrate to the powder cloud. The temperature controlled fluid will also act as a governor for the maximum temperatures achieved in the thinner substrates, depending on its actual temperature and moisture content. It should also be noted that the controlled fluid may offer less electrical resistance than the atmosphere in the surrounding work environment and may be a more favorable electrical conductor for accommodating the electrostatic powder coating process.

[0022] Thermal conductivity within the substrate is not the only reason for temperature anomalies on the process surface. Radiant losses from the heated surface typically exceed convective losses at elevated temperatures, such as 225.degree. F. The emissivity of the wood surface is inherently high, causing the heated part to efficiently emit its thermal energy into space in the form of infrared radiation. This radiant energy loss rapidly decreases the surface temperature of the interface area of the wooden part in process. Lengthy travel time to separate processing equipment allows for a rapid decline of surface temperature because of the combination of radiant, convective and conductive losses. Current processing practices have frequently provided for extended soak times in hot convection ovens in order to satisfy heat sink areas. This can create greater temperature anomalies if the travel time to the next process is lengthy. The thin areas will lose their heat faster because there is less stored energy present due to reduced mass. The rate of thermal losses from the surface of thick and thin areas is equal, but the thin areas attain lower temperatures faster because there is simply less stored energy contained in these less massive areas. Greater temperature differentials between thick and thin parts can then result, causing a chain reaction of events that negatively affect the powder process.

[0023] An objective of this invention is to take advantage of the low rate of thermal conductivity that is inherent to the natural physical properties of the wood material by heating its process surface to uniform temperatures moments prior to the application of the powder coating. Maximum temperature uniformity exists before the heat energy has had time to conduct into the depths of the substrate or to be lost by radiant (and convective) means. This invention permits the highest level of surface temperature uniformity to exist, regardless of thickness, by eliminating unnecessary travel time through inadequately controlled equipment areas during the process. The resulting temperature uniformity preserves the natural and artificial moisture layers that facilitate high electrostatic attraction. The invention also produces ideal surface temperatures, without exceeding the target temperature value, during the application of the powder, which assists in the successful attachment of the powder to the interface area of the substrate.

Problems solved by technology

Since these materials are electrically non-conductive, there are difficulties associated with the consistent earth grounding of the wood that is critical for the electrostatic spraying process.

However, this critical moisture layer is only present for a short period of time.

Further, the moisture that has risen to the target surface is in limited supply, even though significant quantities of moisture still reside within the substrate.

Therefore, the quantity of available moisture that can be rapidly driven to the surface is in very low supply.

Thin recessed areas in wood substrates pose unique moisture problems.

Exposure to dry air over time causes the thin areas to lose their moisture faster than the thick areas.

Initially, both devices will discharge the same voltage, but the larger battery will require more time to completely drain.

In time, this can result in significant differences in total available surface moisture content.

Unfortunately, the moisture content is typically in the range of greatest dissimilarity when it is desirable to powder coat the wood substrate in question.

The unreliably uniform moisture content in the varying wood thickness has prevented the successful powder coating application of many material objects.

This problem is a major inhibitor to the future success of powder coatings on many non-conductive wood products.

Powder processing system designs currently adhere to the "separate equipment" mentality, and are burdened with unfavorable time lags between the heat processes and the application of the powder material to the substrate.

This is due to the significant distances that the parts in process must travel between separate pieces of processing equipment.

However, the substrate experiences a rapid surface temperature drop during its travel through time and space before the powder is actually applied.

This procedure causes a variety of problems for maintaining uniformity of temperatures over the substrate surface.

It can also cause an undesirable depletion of moisture, as well as the expulsion of natural wood resins (sap), from some substrates.

This is true because the absorption ability of IR radiation by the wood surface is high, but its thermal conductivity is considered to be very low.

However, this condition of uniformity will not remain for long periods of time, especially when the substrate must travel a significant distance through space and time to the powder spray application.

It should be noted that the atmosphere, including the chilled (or warm) moisture controlled fluid, does not efficiently absorb IR radiation.

Radiant losses from the heated surface typically exceed convective losses at elevated temperatures, such as 225.degree. F.

This radiant energy loss rapidly decreases the surface temperature of the interface area of the wooden part in process.

Lengthy travel time to separate processing equipment allows for a rapid decline of surface temperature because of the combination of radiant, convective and conductive losses.

This can create greater temperature anomalies if the travel time to the next process is lengthy.

Greater temperature differentials between thick and thin parts can then result, causing a chain reaction of events that negatively affect the powder process.

The area between the preheat oven process and the powder spray application equipment is often designated as a hazardous area, and does not possess the atmospheric thermal properties or processing equipment that is required to maintain the surface temperature of the substrate.

It has been observed that some types of wood products incur damage from thermal expansion, such as cracking or splitting of the substrate, when attempting to achieve high surface temperatures relative to the inner substrate temperature.

These characteristics cause the wood product to experience a large surface temperature increase in a small period of time when exposed to only modest power levels of infrared radiation.

This method is inefficient because the rate of temperature rise, and the resulting thermal expansion differential, is well below the maximum allowable limit.

The artificially high set point temperature that is used to compensate for the aforementioned surface temperature drop during conveyor travel also complicates the thermal expansion aspect in this process.

Current regulations prohibit the installation of certain heat and / or radiation processing equipment within the spray booth area.

There is a general misunderstanding of the nature of electromagnetic radiation in industry today.

A misconception exists that all infrared or ultraviolet radiation loses power if sent over long distances.

However, the energy does tend to disperse at angles that dissipate its concentration, creating the illusion that the rays simply disappeared because of distance.

This means that some of the radiation will not be perfectly aligned in its delivery from a parabolic or elliptical reflector system into the WGCL.

The WGCL will not efficiently absorb the radiation because its internal walls are highly reflective to the subject radiation.

Further, the small quantity of radiation that strikes the internal surfaces of the WGCL are at low angles of incidence that are not favorable for efficient absorption; generally less than a 10.degree. angle of incidence.

This occurs over long periods of time, and has presented some major problems with the efficient maintenance of the processing equipment.

Safety issues have also occurred that are not recognized by safety laws.

However, it has been observed that powder coating materials tend to migrate through the air over time, causing unwanted deposits in normally safe areas.

These deposits tend to accumulate to substantially thick proportions that hamper the maintenance of other radiant processing equipment.

This will cause a sudden contraction or shrinkage of the liquefied surface, while liquefied powder coating of substantially higher temperature resides below the surface.

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